1. Field of the Invention
The present invention relates to catalytic reactors generally and, more specifically, to catalytic support structures for use within a combustor, particularly a combustor for a gas turbine.
2. Related Art
Combustion turbines generally have three main assemblies; a compressor assembly, a combustor assembly and a turbine assembly. In operation, the compressor compresses ambient air. The compressed air from the compressor flows into the combustor assembly where it is mixed with a fuel. The fuel and compressed air mixture is ignited, creating a heated working gas. The heated working gas is then expanded through the turbine assembly. The turbine assembly includes a plurality of stationary vanes and rotating blades. The rotating blades are coupled to a central shaft. The expansion of the working gas through the turbine section drives the blades which, in turn, rotate the shaft. The shaft may be connected to a generator to produce electricity.
Typically, the combustor assembly creates a working gas at a temperature between 2,500xc2x0 F.-2,900xc2x0 F. (1,371xc2x0 C.-1,593xc2x0 C.). At high temperatures, particularly above approximately 1,500xc2x0 C., the oxygen and nitrogen within the working gas combine to form the pollutants NO and NO2, collectively known as NOx. The formation of NOx increases exponentially with flame temperature. Thus, for a given engine working gas temperature, the minimum NOx will be created by the combustor assembly when the flame is at a uniform temperature, that is, there are no hot spots in the combustor assembly. This is accomplished by uniformly premixing all of the fuel with all of the air available for combustion (referred to as low NOx lean-premix combustion) so that the flame temperature within the combustor assembly is uniform and the NOx production is reduced.
Lean-premixed flames are generally less stable than non well-mixed flames, as the high temperature/fuel rich regions of the non well-mixed flames add to a flame""s stability. One method of stabilizing lean-premix flames is to react some of the fuel/air mixture with a catalyst prior to combustion. To utilize the catalyst, a fuel/air mixture is passed over a catalyst material, or through a catalyst bed, causing a pre-reaction of a portion of the mixture to create radicals which aid in stabilizing combustion at a downstream location within the combustor assembly.
Some prior art catalytic combustors completely mix the fuel and the air prior to exposing the mixture to the catalyst. This provides a fuel-lean mixture to the catalyst. However, typical catalyst materials are not active with a fuel-lean mixture at compressor discharge temperatures. A pre-burner is required to heat the air prior to exposing the fuel/air mixture to the catalyst in order for the catalyst to react with a fuel-lean fuel/air mixture to create radicals, which aid in stabilizing combustion. The preburner adds cost and complexity to the design as well as generates NOx emissions. It is, therefore, desirable to have a combustor assembly that burns a fuel-lean mixture, so that NOx is reduced, but passes a fuel-rich mixture through the catalyst bed so that a preburner is not required. The preburner can be eliminated because the fuel-rich mixture contains sufficient mixture strength at compressor discharge temperatures, without being preheated, to activate the catalyst and create the necessary radicals to maintain a steady flame. This is accomplished by providing one flow stream of compressed air richly mixed with fuel that passes over a catalyst bed. A second flow stream of compressed air is isolated from the first flow stream and is used to cool the catalyst bed. The two flow streams of compressed air are combined downstream of the catalyst bed just upstream of the point of ignition.
In such fuel-rich configurations, the catalyst and catalyst support structure temperatures must be controlled to avoid catalyst degradation, excessive substrate oxidation and preignition or flashback conditions. Prior art configurations utilize a tube array geometry wherein cooling air is projected inside the tubes with the catalyst coating applied to the outside of the tubes. Such tube arrays are notoriously susceptible to vibration, degradation, fatigue and fretting induced by base system/engine vibrations and longitudinal (air and fuel-air) and traverse (fuel/air) flow effects. The vibration can result in both tube wall degradation as well as tube to tube sheet joint degradation, e.g., fatigue of brazed or welded joints. Methods applied or proposed for enhanced tube support to overcome the vibration problem have their own shortcomings. For example, intermediate tube supports may cause tube wearthrough due to fretting and can cause counter flow affects and nonuniform mixing. Also, intermediate tube bulges to effect intermediate support or tube flaring to effect end support cause wall thinning and can lead to premature tube failure.
Accordingly, there is a need for a catalyst support structure of improved resilience to vibratory effects. Additionally, there is need for such a structure that will provide for cooling of the substrate and catalyst. Furthermore, such a structure must be readily coatable to host the catalyst and cost effective to manufacture in production quantities.
The foregoing objects are achieved employing a combustion turbine combustor having an elongated catalytic section comprising a tandem arrangement of passages defined in part by first and second side walls, each of the side walls is provided with a plurality of grooves extending in the direction of the elongated dimension of the catalytic section. Alternate passages in the tandem arrangement are formed from a top plate and a bottom plate attached to and affixed together by an undulating member alternately forming ridges and grooves that respectively attach to the top and bottom plates to form a corrugated panel. The undulating member extends substantially from one lateral side of the panel to the other relative to the elongated dimension. Each of the corrugated members are supported at one lateral side within at least one of the grooves in one side wall and on the other lateral side within an at least one of the grooves in the other side wall.
In the preferred embodiment, the one lateral extending side of the top plate is supported within a first groove in the one side wall and the first laterally extending side of the bottom plate is supported within a second groove in the one side wall. Similarly, in this preferred embodiment, the other laterally extending side of the top plate is supported within a first groove in the other side wall and the other laterally extending side of the bottom plate is supported within a second groove in the other side wall. Preferably, one end of the undulating member is supported within either the first or second groove of the one side wall and the other end of the undulating member is supported within either the first or second groove of the other side wall. Each top side of the top plate and bottom side of the bottom plate is coated with a catalyst.
In one preferred embodiment, the corrugated members are connected to an upstream header and the interior of the corrugated members is in fluid communication through the header with a cooling air plenum. A fuel/air mixture plenum is provided in fluid communication with the catalytic lined passages in between the corrugated members. Both the cooling passages and the fuel/air mixture passages are joined at a mixing plenum downstream of the catalytic section to provide a catalytically-enhanced lean-fuel mixture for stable combustion.